Method for retreading vehicle tires

ABSTRACT

A method for retreading vehicle tires. A carcass is provided with a prevulcanized tread and an unvulcanized tie-gum layer between the carcass and tread. A microwave applicator is located above the tread for emitting microwaves to the tire for creating heat energy in the tire. A reflector for microwaves comprising an electrically conductive material is attached in connection to or in the tie-gum layer to achieve high quality rapid vulcanization without heating the carcass.

BACKGROUND OF THE INVENTION

This invention relates to a method for the retreading of vehicle tires.

At the retreading of vehicle tires, a new tread is applied to a carcassand vulcanized therewith by means of a binder layer.

Due to the recent substantial rise of the raw material price, it hasbecome more interesting to retread vehicle tires, above all such forheavy vehicles, but also for passenger cars.

Conventional methods of retreading tires, however, are verytime-consuming and, thus, expensive. Vehicle tires can be retreadedaccording to two main methods. At one of them a carcass is provided witha new prevulcanized tread, after the old tread has been removed bygrinding. At this method a tiegum layer is placed between carcass andtread. The tire thus provided with a new tread is vulcanized in anautoclave at overpressure and at a temperature of about 60° C. to 150°C. It takes about two to five hours according to this method to attachthe tread by vulcanization to the carcass. The heating medium used is aheated fluid, for example steam. There exist different variants of thisschematically described method, for example the so-called bandag system,the so-called Vacuum-Vulc System and the so-called Vulcap system, whichare described in U.S. Pat. No. 4,157,930. At the second one of the saidtwo main methods, an unvulcanized new tread is positioned about acarcass, whereafter the tread as well as the connection between treadand carcass are vulcanized.

In the aforesaid U.S. Patent a method of shortening the vulcanizationtime is described. According to this method, microwave energy is usedfor heating the tiegum layer whereby said layer is vulcanized bothagainst a prevulcanized tread and the carcass. The tie-gum usedaccording to this method has a higher dissipation factor at thevulcanization temperature than the prevulcanized tread, in order therebyto concentrate the heating to the tie-gum layer.

The microwave energy is supplied by an applicator located outside thetire periphery while the tire is being rotated.

By the method according to said U.S. Patent the vulcanization time isreduced substantially. The method, however, shows several disadvantages.One disadvantage is that the prevulcanized tread yet is heated much morethan desired. Furthermore, during the vulcanization process the entirecarcass is heated. These disadvantages are due to the fact, that at thevulcanization temperature, or about the same, the energy is concentratedon the tie-gum layer, because the tie/gum layer then has a higherdissipation factor than the tread and the carcass. In order to rapidlyincrease the temperature of the tie-gum layer, the carcass is filledwith hot water, thereby preventing too great a heat development in thecarcass and tread. By heating the carcass and tread, namely, they aresubjected to ageing, and as a result thereof their service life isreduced.

At the method disclosed in said U.S. Patent, thus, the tie-gum must beselected especially in view of the material of the carcass and tread. Itfurther is necessary, as mentioned, to heat the carcass, and therewiththe tie-gum, prior to the application of microwave energy. This method,thus, subjects both the carcass and tread to undesired heating.

SUMMARY OF THE INVENTION

The present invention offers a method, which is much simpler and morerapid than the one described above. The present invention, besides,implies that a more uniform quality level is achieved, and thatunnecessary heating of carcass and tread is avoided.

The present invention, thus, relates to a method of retreading vehicletires, at which method a carcass is provided with a prevulcanized treadand with an unvulcanized tie-gum layer between the carcass and tread,and at which a microwave applicator is located above the tread totransmit microwaves against the tire for creating heating energy in thetire.

The invention is characterized in that a reflector for microwaves,comprising an electrically conductive material, is attached adjoining orin the tie-gum layer, thus between the carcass and the prevulcanizedtread. The invention of a divisonal application of which the presentapplication is the parent relates to an apparatus for the retreading ofvehicle tires, by which apparatus a carcass is provided with aprevulcanized tread and an unvulcanized tie-gum layer between thecarcass and tread, and at which by means of a microwave applicatorlocated above the tire microwaves are emitted against the tire forcreating heat energy in the tire. The apparatus is characterized in thatit comprises a reflector for microwaves, which reflector comprises anelectrically conductive material and is intended to be attached at or inthe tie-gum layer.

BRIEF DESCRIPTION OF THE DRAWING

The invention is described in greater detail in the following, withreference to the accompanying drawing, in which

FIG. 1 is a schematic cross-section of a tire, partially in an explodedview,

FIG. 2 is a schematic cross-section of a tire in the state prevailing atvulcanization,

FIG. 3 is a cross-section of an outer portion of a tire on an enlargedscale,

FIG. 4 is a cross-section of a web-shaped tie-gum according to oneembodiment of the invention, and

FIG. 5 shows a portion of a tire in a holding device and a portion of amicrowave applicator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 a cross-section of a tire 10 is shown schematically, whichtire comprises a tread 1, a tie-gum 2, a carcass 3, an inner tube 4 anda valve 5. The tire is mounted on a rim 6. On a tire to be retreaded theold tread is removed by grinding so that only the carcass 3 remains.Thereafter a tie-gum layer 2 is applied on the outside of the carcass 3,and a completely vulcanized tread 1 is applied on the tie-gum layer 2.On a tire 10 thus prepared for vulcanization a so-called envelope 7 isthreaded, i.e. a casing enclosing the outer portion of the tire 10. Thesaid envelope has the object to maintain the tread 1 and the tie-gumlayer 2 in place relative to the carcass 3 while the tie-gum layer isbeing vulcanized. On said envelope 7 a valve 8 is located for evacuatingair between the envelope 7 and tread 1.

At known vulcanization methods the tire 10 enclosed by an envelope 7 isinserted in an autoclave, and the entire tire thereafter is heated, forexample by steam, while overpressure prevails both in the autoclave andin the inner tube 4 of the tire. When the vulcanization temperature hasbeen achieved and maintained for a sufficient time, the tire is cooled.

At the method described in the aforesaid patent, the tire 10 is placedinto an autoclave with an overpressure prevailing therein. The innertube of the tire is filled with hot water in order to heat the tie-gumlayer 2 from the inside and pressure is applied thereon. Thereafter thetie-gum layer 2 and also the tread 1 are heated by means of microwaveenergy, which is fed through the tread 1 by a microwave applicator whilethe tire 10 rotates. This lastmentioned method has severaldisadvantages, viz. as mentioned that the carcass 3 and also the tread 1are unnecessarily much heated, and that the heating of the carcass withhot water is time-consuming and cost-increasing. When microwave energyis used for vulcanization, as set forth in U.S. Pat. No. 4,157,930, inthe case of steel radial tires the steel cord is a reflector for emittedmicrowaves. The distance, however, from the surface of the tread to thesteel cord varies substantially across the width of the tread, and thesteel cord has not the same width as the tie-gum layer. Hereby, thus,the heat is distributed non-uniformly in the tie-gum layer, and at thesame time difficulties arise in heating the outer portions of thetie-gum layer to a sufficient extent. This implies substantialdifficulties in manufacturing a fully satisfactory product.

According to the present invention, the tire 10 is prepared as describedabove inclusive of threading-on an envelope 7, but with the essentialdifference that a reflector for microwaves, comprising an electricallyconductive material, is attached in connection to or in the tie-gumlayer 2, i.e. between the carcass 3 and the prevulcanized tread 1.

At a first preferred embodiment of the invention, a web-shaped tie-gum 2is used which includes electrically conductive material and is attachedbetween the carcass 3 and the tread. The electrically conductivematerial preferably contains wires 9, which are arranged in parallelspaced relationship with each other. According to a preferredembodiment, the wires 9 extend in the circumferential direction of thetire 10, i.e. in the longitudinal direction of the web-shaped tie-gum.

FIG. 4 is a cross-section of a web-shaped tie-gum identical with thecross-section according to FIG. 3. In FIG. 4 wires 9 are shown, whichare cast-in in an unvulcanized tie-gum layer 2 and uniformly distributedacross the width of the tie-gum web 2.

A tie-gum layer 2 normally has a thickness of about 1.5 to 2 mm. It wasfound that in that case wires with a diameter of about 0.10 to 0.20 mmare suitable. Suitable wire materials are steel and copper, but alsoother electric conductors can be used. The distance between the wiresrelative to each other depends on the frequency of emitted microwaves.Said distance should be about λ/12 where λ is the wave length of emittedmicrowaves. A microwave frequency suitable for this purpose is 2450megacycles per second. When this frequeny is used, it was found suitableto arrange the wires at a spaced relationship of 5 mm to 15 mm,preferably of 10 mm.

It is, of course, not necessary to cast-in such wires in theunvulcanized tie-gum layer, but the wires may be attached in some otherway.

According to a second preferred embodiment of the invention,electrically conductive material in the form of wires or a net isattached either on top of the carcass 3 prior to the application of thetie-gum layer 2 or on top of the tie-gum layer after the applicationthereof. In the lastmentioned case the electrically conductive materialcan be attached in a first step to the lower surface of the tread 1.

The aforesaid embodiments have in common that the material provided asreflector for microwaves is caused to cover a width corresponding to thewidth of the tie-gum layer 2. This ensures substantially equalconditions, from a microwave aspect, across the entire tie-gum layer.

The present invention, as mentioned in the introductory part, alsorelates to an apparatus for carrying out said method.

The apparatus comprises a reflector for microwaves, which includes anelectrically conductive material and is intended to be attached to or inthe tie-gum layer. The structural design of the apparatus and itsdifferent embodiments are clearly apparent from the above description ofsaid method according to the invention.

By attaching wires 9 of electrically conductive material in or at thetie-gum 2 the electric field of intensity, and therewith the microwaveenergy, are concentrated to the position of the wires. The tangentiallylocated component of the electric field of intensity (E_(t)) isshort-circuited in that the wires 9 extend in the circumferentialdirection of the tire 10. As a result thereof, the radially locatedcomponent (E_(r)) is at maximum at the wires 9, see FIG. 5.

The energy density of microwave energy, thus, increases from the surface11 of the tread 1 inward to the tie-gum layer 2. This implies that theheat development in the tread 1 increases from the surface thereof tothe transition of the tread to the tie-gum layer 2. Hereby the tread 1is heated most at the tiegum layer 2. The heat developed is transferredconductively to the tie-gum layer 2, which thereby is heated tovulcanization temperature. Heat energy, of course, also is developeddirectly by the microwave energy in the tie-gum layer 2, to an extentcorresponded by the chemical composition of the tie-gum layer.

Vulcanized rubber normally has a dissipation factor exceeding that ofunvulcanized rubber at the vulcanization temperature. In the aforesaidU.S. Pat. No. 4,157,930, however, tie-gum compounds are stated whichhave a higher dissipation factor than a vulcanized tread.

By attaching a reflector such as electrically conductive wires in thetie-gum layer, thus, the tie-gum layer is heated to vulcanizationtemperature substantially independent of the microwave properties of thetie-gum. At the same time, the tread 1 is heated only partially and mostnear the tie-gum layer 2.

When vulcanization is carried out according to the present invention,the tire is positioned into an autoclave 14 of known conventional type.The rim 6 is mounted by a holding member 12 in a driven rotary shaft.One or more microwave applicators 13 are provided in the autoclave 14 inconnection to the periphery of a positioned tire 10. The autoclave 14 isonly partially shown in FIG. 5.

The interior of the autoclave 14 is pressurized with air, for example tothe pressure 7 bar, and the inner tube of the tire 10 is pressurizedwith air to, for example, 9 bar. The air in the autoclave 14 as well asin the tire 10 is of room temperature. Thus, neither the interior northe exterior of the tire is preheated. Subsequent to the pressurizationof the autoclave 14 and tire 10, the tire is rotated by said shaft andmicrowave energy is supplied against the tire by means of theapplicator(s).

As described above, heat is hereby developed uniformly distributed aboutthe tire. As an example can be mentioned, that the time for heating thetie-gum layer 2 of a truck tire to the vulcanization temperature, about140° C. to 150° C., is about 7 minutes. At this temperature thevulcanization of the tie-gum layer 2 is carried out during 10 to 12minutes. The tire is thereafter removed from the autoclave 14, and saidenvelope 7 is taken off. The tire is then ready.

By means of the present method the entire carcass is substantiallymaintained cold during the entire process. Due to the fact that the heatdevelopment to its greatest part occurs in connection to the tie-gumlayer 2, also the outer portions of the tread are maintainedsubstantially cold. In spite of these advantages, the vulcanization timeis as short as 17 to 20 minutes, which is particularly essential forreducing the retreading costs, compared to known methods.

The completely vulcanized tread 1, at substantially all retreadingmethods, has a substantially constant thickness and a lower surface 15,which bellies only slightly. By the present method, at which thereflector for microwaves is attached at or in the tie-gum layer, thepenetration depth for the microwaves through the tread 1 to thereflector is substantially constant across the width of the tread 1 ortie-gum layer 2, which in its turn implies that the heat developed issubstantially uniformly distributed across the width of the tie-gumlayer 2.

When microwave energy is used for vulcanization, as described in theafore-mentioned patent, the tie-gum layer as described above is heatednon-uniformly across its width. It is obvious that the present inventionrenders rapid vulcanization possible substantially without giving riseto a heating of the outer portions of the carcass and tread. Thevulcanization, in addition, has a very uniform and high quality.

It further is obvious that the method can be applied to any type oftire, for example to tires with textile cord as well as to tires withsteel cord and to tires of different dimensions and structural designs.Even tires with a cord of glass fibres or Kevlar® can be retreadedaccording to the present invention.

The invention, of course, can be varied in many ways. The reflector, forexample, can be designed as a net of some kind, as wires intersectingeach other, narrow strips etc.

The reflector also may consist of a tie-gum, which has been providedwith a sufficient amount of a pulverous electrically conductive materialto become electrically conductive at the microwave frequencies intended.

The reflector also can be applied immediately prior to the retreadingprocess or it may be prefabricated, for example, by being cast-in in thelower surface of the tread, as mentioned above.

In the foregoing, pressures and dimensions have been stated by way ofexample. Pressures, dimensions and microwave applicators, of course,must be selected and designed in view of the tires to be retreaded.

The present invention, therefore, must not be regarded restricted to theembodiments set forth above, but can be varied within the scope of theattached claims.

What is claimed is:
 1. A method of retreading vehicle tires comprisingthe steps of providing an assembly comprising an unvulcanized tie-gumlayer between a prevulcanized tire carcass and a prevulcanized tread,providing an electrically conductive reflector for microwaves inassociation with the tie-gum layer between the carcass and the tread,and locating a microwave applicator above the tread and emittingmicrowaves having an electrical field with tangential and radialcomponents to the assembly, said reflector being of a widthcorresponding to the width of the tie-gum layer and arranged to shortcircuit the electric field component in the tangential direction of theassembly, so as to maximize the radially directed component of theelectrical field to heat the tie-gum layer to vulcanization temperaturesubstantially independent of the microwave properties of the tie-gumlayer or carcass.
 2. A method as claimed in claim 1 in which the tie-gumlayer is web-shaped and is attached between the carcass and the tread.3. A method as claimed in claim 1 or 2 in which the tie-gum layerincludes wires of electrically conductive material arranged in parallelspaced relationship.
 4. A method as claimed in claim 2 in which thetie-gum layer includes wires of electically conductive materialextending in the circumferential direction of the tire and thelongitudinal direction of the web-shaped tie-gum layer.
 5. A method asclaimed in claim 1 in which the electrically conductive reflector isformed of wires and is attached on top of the carcass prior toapplication of the tie-gum layer.
 6. A method as claimed in claim 1 inwhich the electrically conductive reflector is formed of wires and isattached on top of the tie-gum layer after application thereof.